Charles e



(No Model.)

C. E. SCRIBNER. TEST CIRCUIT POR-.MULTIPLE lSWITCHBOARDS..

No. 502,774. Patented Ang. 8,1893

W s /j Waff@ UNITED STATES PATENT OEETCE.

CHARLESv E. SCRIBNER, OF. CHICAGO, ILLINOIS, ASSIGNOR TO THE WESTERN ELECTRIC COMPANY, OF SAME PLACE.

TEST-CIRCUIT FoR M u LrlPt-E f-swlToH BOARDS.

SPECIFICATION'vforming part of Letters Patent No.v`5(`)2,774, dated August 8, 1893.

' Appiittiqn nea July 7.1891.- seritlNo. 398,718. (No model.)

- description, reference being had to the accompanying drawing, forming a part of this specification. Y

My invention relates toY testing systems fo multiple switch boards ot telephone ex-v` changes; its object is to provide means where- -by stray currents in the telephone lines arising from causes external to thel telephone system may be prevented from flowing through the test circuit and causing false test signals.

My invention relates more speciiically to that class of switch board circuits in which the frames or test rings of the spring jacks of a line are connected to one of the line wires from the substation, and in which a portion of this conductor is included in the test circuit which'is completedwhen a test of a busy line is made. It is a fact of common experience that when the line wires of such asystem extend parallel or in proximity to con` ductors carrying heavy and varying currents, such as electric light and railway lines, currents are induced upon the telephone lines, which escape to earth through the testing apparatus when a test is made, and cause false and misleading signals.

In my invention I include aV condenser in that line wire which is connected to the test rings of the spring jacks, between thelinewire and the frame of the rst spring jack. The condensertransmits the undulatory-or alternating telephonie. currents undiminished in volume, but serves as acomplete rupture ofthe circuit to approximately steady'currents. It is still possible to' employ the line which includes the condenser, as the return line for the calling circuit, since the rapidly alternating current from the calling-magneto Will be transmitted throughit; but Ipret'er to arrange the apparatus at the substations and the individual annunciators so that the earth shall act as return 'for the calling currents. In'a telephone exchange system thus protected fromy external currents I am enabled to employ any of the Well knowntest systems Whose test circuits are local; I prefer, however," to include a rheotome at some part of the test circuit, to produce a tone in the telephone of the testing operator when a line whichis already in use is tested.

My invention may be applied indifferently to grounded and to metallic circuits.

My invention is-illustrated in the accompanying drawing, and will bevmore readily I understood by reference thereto.

In the drawing I have lshown two substations connected by lines, one line wire of each of 'which includes a condenser, to their respective spring jacks and annunciators at two sections of multiple switch board. The switch tus for performing theV various operations involved in establishing communication between two substations, comprising connecting cords and plugs, calling and listening keys, and means for determining at one switch board whether or not aline is in use at another board. l

Thisapparatus is all of well known construction an'd operation; hence I will describe it only brieiy. Y

Thesubscribers. apparatus at substation No. l comprises a signal bell, which is normally included in a branch connection from the line ct to earth, a calling generator which is automatically included in the same branch connection when the generator is operated,a telephone set, and a switch whereby when the telephone set is in use the call bell and generator are disconnected from the line a, and

the telephone set is included in a branch connection between linesa and a. Thus a call signal is sent over line a, to return throughI the earth, while the telephonie or'talking circuit is over line ct, returning by line a. The lineaJ vmay be traced to aA spring jack a2 at the rst switch board, at the left of the drawing, through its line spring and contact anvil, thence to and through the line spring and contact of jack a3 at the second board, thence to answering jack a6 Where it is connected to to the line spring, thence through annunciator a4 to earth.- Line a may be tracedy to condenser a5, thence to the spring jacksa2 as a6 Vboard I have shown provided with appara- IOO where it is connected to the frames or test rings of the jacks.

The circuit of station 2 isshown as grounded. The apparatus at the substation differs from that at station l, in that the telephone switch, when the telephone is in use, disconnects the calling apparatus from the line b and includes the telephone set in a connection from line to earth. The line ZJ may be traced to the line spring of its spring jack h2 at the first board, thenceto jack b3, at the second board, thence returning to answering jack b at the first board, thence through annunciator b4 to earth. The telephonie current from station 2, is provided with a metallic circuit throughout the switch board system. The telephonie current returning from station 2, finds circuit through resistance 127, condenser b5 to line b', which is connected to the test rings of spring jacks h2 h3 h6.

The operators outfit comprises, to describe it more specifically, the two loop plugs c c', each provided with a tip and sleeve adapted to make contact, the tip with the line spring and the sleeve with the frame or test ring of a spring jack into which it may be thrust. Corresponding contact pieces are electrically connected together, the tips through a condenser, and the sleeves directly. Two calling keys d e are included in the circuit between the two connecting plugs, one on either side of the condenser, and each is adapted, when depressed, to disconnect both contact pieces ot' one of the plugs from those of the other, and to connect them to the two poles of a calling generator. Branches are extended from the conductors joining,r corresponding parts of the plugs to the contact anvils of a listening key f which is adapted, when its lever is placed in the vertical position as shown, to connect the two terminals of a telephone set g to the contact anvils; whereby the telephone set is included in a branch or shunt circuit between two subscribers placed in communication by means of connecting plugs c c. Another branch connection is ext-ended from the conductor joining the sleeves of the connecting plugs, through a test battery h to earth. A self induction or retardation coil z' is included in the branch between the battery and the sleeve of the plug, in order that several pairs of connecting 'plugs may be similarly connected to the same test battery without crossing together the different pairs of plugs, it being understood that no appreciable portion of the telephonie current will pass through the retardation coil. Obviously when the plug c is thrust into any spring jack, the sleeve of the plug coming in contact with the test ring of the spring jack, a difference of electrical potential is set up between the test rings of all the spring jacks of that line and the earth. If now a connection is completed from :earth to the test ring of a spring jack of that line at another board, a current will be established in the circuit and will give a signal in an appropriate responsive device included in the circuit. In the system shown, the plug c' is used as this second branch connection, to complete the circuit. A connection is extended from the middle of the operators telephone coil g' to earth; consequently when the tip of plug c is applied to the test ring of a telephone line, to which a ,connection already exists, a current nds circuit from tip of plug c through the listening key f, one half of the coil ot operators telephone g, to earth; the telephone g serves as a responsive device. A rheotome 7s is included in the branch connection from telephone g to earth, which serves to interrupt the current flowing through the telephone, and the response becomes a musical tone.

It will be obvious that if the condenser in the line circuit were charged statically from an induced current on the portion of the line between the substation and the central ofiioe when the test was made, it would be discharged at the tirst touch of the testing plug, and could produce no further signal in the telephone,that is, no continuous sound.

I will now proceed to describe the operation of the system shown. Suppose that subscriber at substation l desires to communicate with subscriber at station 2. Turning his generator, a current Hows over line a, through the spring jacks a2 0,3, and annunciator a4 to earth. The shutter of the annunciator is released, and falls, indicating to the operator that a connection is desired by subscriber at station 1. She inserts plug c into answering jack a and throws the listening key f into the position shown, whereby the telephone set g is connected to line. Subscriber at station l has meanwhile removed his telephone from its hook, and is therefore in communication with the operator at the second board; the circuit between them is a complete metallic circuit. The operator having learned that a connection with station 2 is desired, proceeds to test that line" to determine whether the line is already in use at some other board or not. Applying the tip of plug c to the test ring Z of jack b3 at her board, if she hear a tone in her telephone upon each touch of the plug to the test ring, she knows that the line is already in use. Suppose, however, that she hears no sound; this is indicative that the line is not in use, since no source of electro motive force is in connection with the lines. She now thrusts plug c into jack b3, whereby the two substations are placed in connection, and depresses key e, sending a calling current over line b through the signal bell at station 2. When subscriber at station 2 has removed his telephone from its hook, the two telephone sets are in connection over a circuit which is metallic from station l to the resistance 117, thence through earth to substation 2.

Obviously, electro motive force induced upon the line wire a could produce no further eiteet than to charge the condenser a5; when connection was made to earth, as in IOO -extending from a substation, calling apparatus at the substation included in a connection between lineal and earth, a'nd a switch adapted to connect the line c, through a telephone set to line a', spring jack switches having line springsconnected to line a and test rings connected to a conductor, a condenser included between the said conductor and line 0,', and an annunciator included in a branch connection from line a to earth.

3. In combination, two telephone lines exz 5 tending from a substation, calling Vapparatus at the substation normally included 1n a branch from one of said lines to earth, a switchk adaptedto disconnect the said line from earth andto connect it through a telephone to the other line, and at the exchange, spring jacks having line springs electrically connected to that line which is normally grounded at the substation, and test ringsv electrically connected through a condenser to the other line, a branch connection extending from one of the test rings through a source of electricity to earth, and another branch connection extending from another of the test rings through an electro magnetic responsive device to earth, 4o

substantially as and for thepurpose described In witness whereof I hereunto subscribemy name this 25th day of April, A. D. 1891.

CHARLES E. SCRIBNER.

Witnesses:

.ELLA EDLER, GEORGE L. CRAGG. 

